1. Field of the Invention
This invention relates to metal hydrides, and in particular, to processes involving such hydrides.
2. Art Background
Metal hydrides are used in a variety of industrial applications. Although there are many such applications, possibly the most prominent is the use of metal hydrides in batteries. For example, secondary nickel-metal hydride batteries employ lanthanum nickel hydride (or alloy modifications) or other intermetallic hydrides in the negative electrode. A variety of other uses involving energy storage and transfer have been described. Irrespective of the application, a crucial step in preparation is activation of the intermetallic. Activation is achieved, for example, by repeatedly reducing the metal such as LaNi.sub.5 to the corresponding hydride with H.sub.2 gas at high pressure and/or temperature followed by removal of hydrogen at lower pressures.
This cyclic process, generally denominated activation, is believed to serve a number of purposes. Each reduction to the hydride 1) removes reducible surface oxides which tend to interfere with the functioning of the material in the ultimate desired application, 2) induces a reduction in particle size resulting from an increase in volume that causes fracture of the metal particles, and 3) changes the structure and/or composition of the material and/or surface of the metal. Any one or a combination of these three effects is generally employable to increase the rate of reversible hydrogen reaction and, thus, enhance the operation of the material for applications such as batteries or hydrogen storage.
Methods of activation include 1) hydriding with hydrogen gas at high temperatures and pressure; 2) hydriding with chemical hydriding agents; 3) etching with hot hydrofluoric acid or KOH; 4) pulsing the material between hydriding and dehydriding potentials in electrochemical cells; and 5) conventional battery cycling of metal hydride electrodes. However, activation of hydrides has most widely been performed by the first process, i.e., activation, at relatively high pressures (up to 1000 psi) and temperatures as high as 450.degree. C., by subjecting the metal directly to hydrogen gas. Clearly, although such conditions are not prohibitive to commercial use, they require relatively large expenditures for suitable equipment. Thus, an alternative to high pressure reaction of hydrogen gas with the corresponding metal would be quite desirable.
Additionally, metal hydrides, as they are used in batteries such as nickel/metal hydride batteries, have been observed to undergo serious corrosion. (See T. Sakai et al., Journal of the Electrochemical Society, 134, p. 558 (1987).) This corrosion substantially reduces the lifetime of such batteries. It has been reported (see T. Sakai supra), that plating the metal hydride with a metal such as copper, allows the hydride to function as an electrode within the battery and yet prevents or substantially reduces the objectionable corrosion. A metal coating also acts as an oxygen barrier protecting the hydride alloy surface from oxidation and as a microcurrent collector for the charge transfer reaction occurring on the surface. Additionally, a metal coating aids in heat removal, improves electrical conduction, and improves the mechanical stability of the electrode. However, consistently producing a uniform coating of metal on the hydride is difficult to accomplish. Therefore, a highly activated metal hydride uniformly plated with a metal such as copper would be quite desirable.